Iraqi security forces and aligned Shiite militias continued Wednesday to encircle the Islamic State-held city of Tikrit in the third day of a major operation to retake Saddam Hussein’s hometown and the scene of a major victory for the militant group last June.

An official in the Salahadeen Operations Center, which is running the operation on the main north-south highway linking the capital of Baghdad to the Islamic State-held city of Mosul, said that although the operation, which began Monday, has been proceeding slowly because of huge numbers of roadside bombs, suicide bombers and well-entrenched snipers, the government forces had cut a key resupply artery to the town and retaken control of two small oilfields.

Reached by telephone, the official, Capt. Hayder Kadhim said that in the areas of al Dweir and al Aram, villages to the south of Tikrit, which is about 120 miles north of Baghdad, the security forces had been pushed back by “the density of mines and the deployment of snipers on buildings’ roofs.” Nonetheless, Iraqi aircraft are bombing those areas, he said, to clear the way for engineering teams to dismantle roadside explosive devices and car bombs.

The lack of coalition air support – U.S. planes and their allies regularly bomb Islamic State positions in both Iraq and Syria – has continued with U.S. officials admitting that the Iraqis did not inform the Americans in advance of the operation and have yet to request assistance.

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The overwhelming presence of Iranian-backed Shiite militias – which are being publicly led by top Iranian officers from the Revolutionary Guard – has made multiple U.S. officials say they are deeply concerned that the battle has taken a sectarian tenor, as Shiite Muslim irregular troops commanded and equipped by Iran have taken the lead in the heart of Sunni Iraq, where suspicion about both the Shiite-dominated government and Iran runs deep.

Multiple unconfirmed but credible reports have indicated that Iranian troops along with Lebanese Hezbollah allies are helping coordinate the attacks and operating artillery, rocket-launcher systems and Iranian-made surveillance drones.

Army Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said Iranian involvement in the Tikrit campaign was larger and more overt than in past cases, but he added that it could turn out to be “a positive thing.”

“This is the most overt conduct of Iranian support, in the form of artillery and other things,” he said in response to questions from members of the Senate Armed Services Committee. “Frankly, it will only be a problem if it results in sectarianism.”

Capt. Kadhim said the security forces had managed to take control of two oilfields and push into the al Askari district just a few miles from the center of town, as security forces and militias working from the north had fought their way down from Tuz Khor Mato to cut off the ability of the Islamic State to reinforce its fighters inside Tikrit itself.

The progress, however, does not mean there will be a quick end to the operation. Kadhim warned that the first days of fighting had been designed to encircle the city and cut off its supply routes, but that the actual entry into the city of Tikrit itself would pose an enormous challenge for security forces.

“The security forces are advancing significantly, but operations may continue until next month,” he said. “The security forces, even as they enter the city center of Tikrit, will face the street fights and suicide bombers hiding inside those streets and homes. Many of the suicide bombers have been seen and they are carrying out suicide operations on Iraqi security forces by wearing the same (Iraqi) uniforms, which form a kind of confusion.”

It’s this sense that the fighting will evolve into a slow, bloody, house-to-house urban fight among civilians that has people – mostly Sunnis stuck between radicals on both sides – terrified and attempting to flee. Even now, there are reports of significant civilian casualties from sources in nearby Samara, a majority Sunni town with a strong Shiite militia presence because of an important religious shrine.

Samara has been put under an extensive curfew, according to local residents, because of the unsolved and apparently sectarian murders of two Sunni merchants in the city’s market on Tuesday, which spawned anger toward the Iraqi security forces and militias, which are widely suspected in the incident by the local population.

“(The backlash) made the security forces fear the people and their reaction,” said one resident, who requested anonymity because of the danger of being identified by name. The resident said that with the heavy bombardment of nearby Tikrit, and with Samara being used as a key base for more than 20 Iraqi army and Shiite militia units, the mostly Sunni locals are both fearful and outraged by what the witness said was a non-stop stream of casualties, both military, Islamic State and civilian.

“I would like to assure you that the Samarra hospital is filled with bodies belonging to security forces and the (Shiite militias) and the number of wounded is very large,” said the source, who was at the hospital on Tuesday. The government shelling “has also left tens of martyrs of women and children because they are trapped and cannot get out of those cities.”

The resident said that the Islamic State was refusing to let civilians flee the areas of its control and that most civilians – primarily Sunni – are terrified of the security forces and the Shiite militias because of the non-stop rhetoric used by state television and Shiite fighters that the operation in Tikrit is revenge for the massacre of at least 1,000 air force cadets and volunteer fighters captured by the Islamic State in June. Their executions were videotaped and distributed widely on social media, and Shiite and Iraqi government leaders often reference the massacre, leading to widespread concerns about revenge killings.